Single-frequency narrow linewidth 2 μm fiber laser

ABSTRACT

A compact single frequency, single-mode 2 μm fiber laser with narrow linewidth, &lt;100 kHz and preferably &lt;100 kHz, is formed with a low phonon energy glass doped with triply ionized rare-earth thulium and/or holmium oxide and fiber gratings formed in sections of passive silica fiber and fused thereto. Formation of the gratings in passive silica fiber both facilitates splicing to other optical components and reduces noise thus improving linewidth. An increased doping concentration of 0.5 to 15 wt. % for thulium, holmium or mixtures thereof produces adequate gain, hence output power levels for fiber lengths less than 5 cm and preferably less than 3 cm to enable single-frequency operation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of priority under 35 U.S.C. 120 to andis a continuation in part of U.S. application Ser. No. 10/056,830entitled “Rare-Earth Doped Phosphate-Glass Single-Mode Fiber Lasers”filed on Jan. 24, 2002, now U.S. Pat. No. 6,816,514, the entire contentsof which are incorporated by reference.

GOVERNMENTAL RIGHTS

This invention was made with Government support under ContractNNL05AA94P awarded by NASA. The government has certain rights in thisinvention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to fiber lasers and more specifically to a singlefrequency 2 μm fiber laser with narrow linewidth (<100 kHz andpreferably <10 kHz) formed from glass fibers doped with Thulium andHolmium oxides and mixtures thereof.

2. Description of the Related Art

Rare-earth doped glass fiber lasers were first proposed in the 1960s andhave received considerable attention in the 1980s for potentialapplications in optical communication. For laser emission to occur, theactive fiber is placed inside a resonant cavity. The optical feedbackcan be provided simply by the reflectivity of the end facets, bymirrors, by distributed feedback Bragg (DFB) gratings, or by distributedBragg reflectors (DBR), or by constructing a ring cavity structure.Laser emission occurs when the total gain overcomes the losses in thecavity. Hence, a minimum gain has to be achieved to reach the laserthreshold condition. Typical fiber lasers lase in a great number oflongitudinal modes in single transverse mode optic fibers, the so-called“single mode fibers”. A “single frequency” fiber laser lases at a singlelongitudinal mode.

Most telecommunications applications operate at a wavelength of 1.55 μmto take advantage of the absorption characteristics of silica fiber.1.55 μm single-frequency lasers using Er³⁺ doped silica or silica-basedfibers are described in G. A. Ball, W. W. Morey, W. H. Glenn, IEEEPhotonics Technology Letters, Vol. 3, No. 7, July 1991; W. H. Loh et al.Journal of Lightwave Technology, Vol. 16, No. 1, pp. 114–118 January1998; and U.S. Pat. Nos. 5,305,335 and 5,237,576.

2 μm lasers are important because of the high transmission properties inair at that wavelength. Applications such as seeder lasers, LIDAR,optical heterodyne systems, nonlinear frequency conversion, coherentsatellite communication, and distributed sensing require a robust 2 μm,e.g. typically 1.8–2.1 μm, single frequency narrow linewidth fiberlaser. Single-frequency operation with a linewidth less than 100 kHz,preferably less than 10 kHz, provides both the resolution and longcoherence needed by these applications. The 2 μm laser would preferablyhave sufficient output power without the need for a booster amplifier inorder to maintain the high signal to noise ratio.

Current 2 μm single frequency laser technology is limited to solid statelasers, which are either configured to use an intracavity diode pumpedetalon in conjunction with an external reference etalon (U.S. Pat. No.5,457,706 to McGuckin) or as a non-planar ring oscillator (NPRO) in thepresence of a sufficiently strong magnetic field (U.S. Pat. Nos.4,578,793 and 5,043,996).

Current 2 μm fiber laser technology is limited to single transverse modeperformance. J. Y. Allain et al Electronics Letters 25, 1660 (1989)reported on a single transverse mode laser operation at 1.88 and 2.35 μmusing a 150 cm long thulium doped fluorozirconate fiber. J. N. Carter etal Electronics Letters 26, 599 (1990) reported on a cw thulium dopedfiber laser that emits at 1.97 μm using the ³H₄-³H₆ transition in amultimode fluoride glass fiber. Cladding pumped Tm doped silica fiberlaser at 2 μm were reported by R. A. Hayward et al Electronics Letters,Vol. 36, No. 8, pp. 711 (2000), S. Jackson et al, Journal of OpticalSociety of America B, Vol. 16, No. 12, pp. 2178 (1999), Optics Letters,Vol. 23, No. 18, pp. 1462 (1998), and W. A. Clarkson et al, OpticsLetters Vol. 27, No. 22, pp. 1989, (1989). Mode-locked thulium fiberlaser was described by R. C. Sharp et al Optics Letters, Vol. 21, No.12, pp. 881, (1996). Q-switched high peak power operation at 2 μm in Tmdoped silica fiber was reported by A. F. El-Sherif et al Optics Letter,Vol. 28, No. 1, pp. 22, (2003). Fiber laser operation in thuliumsensitized holmium doped silica fibers were reported by K. Oh, et alOptics Letters, Vol. 19, No. 4, pp. 278 (1994), C. Ghisler et al IEEEJournal of Quantum Electronics, Vol. 31, No. 11, pp. 1877, (1995), andS. D. Jackson et al IEEE J. of Quantum Electronics, Vol. 34, No. 9, pp.1578 (1998).

SUMMARY OF THE INVENTION

The present invention provides a compact single frequency, single-mode 2μm fiber laser with narrow linewidth.

The 2 μm fiber laser includes a gain fiber formed of a low phonon energyglass host, either fluoride based or an oxide-based multi-componentglass such as germanate or tellurite, that is doped with triply ionizedrare-earth thulium or holmium oxide or mixtures thereof. Erbium andytterbium may be used to absorb and transfer pump energy to the activeions. Fiber gratings are formed in sections of passive silica fiber andfused to the gain fiber to form the resonant cavity. A dopingconcentration of 0.5 to 15 wt. % for either thulium or holmium oxides ormixtures thereof produces adequate gain, hence output power levels forfiber lengths less than 5 cm and preferably less than 3 cm to enablestable single-frequency operation. Formation of the gratings in passivesilica fiber both facilitates splicing to other optical components andreduces noise thus improving linewidth. The pump can be a multimode orsingle mode laser. In the case of a single mode laser, the use of apolarization maintaining (PM) fiber in the pump source further improveswavelength stability, linewidth and vibration/acoustic sensitivity.

In another embodiment, the gain fiber is a polarization maintaining (PM)fiber, which reduces the noise level in the laser and improves outputpower stability. As a result of their specific local environment, theactive ions experience an anisotropy that leads to polarizationdependent gain, which means that the gain spectra for two differentpolarization components are not the same. Combining polarizationselective feedback with a polarization maintaining active fiber reducesthe low frequency noise that results from random polarizationfluctuations inside the laser cavity.

These and other features and advantages of the invention will beapparent to those skilled in the art from the following detaileddescription of preferred embodiments, taken together with theaccompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a single-frequency fiber laser in accordance withthe present invention;

FIG. 2 is a plot of the absorption spectra of thulium and holmium dopedglasses;

FIG. 3 is a diagram of the longitudinal mode spacing and gratingbandwidth that produce a single-frequency output;

FIG. 4 is a sectional view of the gain fiber;

FIGS. 5 a and 5 b are diagrams of a packaged single-frequency fiberlaser including temperature control and vibration isolation; and

FIG. 6 is a diagram of a pump source with a PM fiber.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a compact single frequency, single-mode 2μm fiber laser with narrow linewidth, less than 100 kHz and preferablyless than 10 kHz. As shown in FIGS. 1 through 4, a 2 μm fiber laser 10includes a single mode gain fiber 12 formed of a low phonon energy glasshost doped with triply ionized rare-earth thulmium or holmium oxide ormixtures thereof. Erbium or ytterbium dopants may also be used to absorband then transfer pump energy to the active ions. The low phonon energyglass host is selected from either an oxide-based multi-componentgermanate or tellurite glasses or a fluoride based glass. These hostglasses have sufficiently low phonon energy to slow down themulti-phonon relaxation process. Narrowband and broadband fiber gratings14 and 16, respectively, are formed in sections of passive silica fiber18 and 20 and fused to the ends of gain fiber 12 to form a resonantcavity that provides the feedback necessary to sustain laser operation.The reflectivity of the broadband grating 16 should be as close to 100%as possible. The reflectivity of the narrowband grating 14 is less than100%, suitably 30–90% depending upon the pump power and target outputpower, such that power can be removed from the cavity. Alternately, thebroadband grating could be used to output couple the laser energy.

The low phonon energy glasses (germanate, tellurite and fluoride)exhibit high quantum efficiency for the 2 μm transition. These glassesalso support the high doping concentrations required to providesufficient gain in short fiber lengths to achieve both single frequencyoperation and narrow linewidths. The holmium oxide (Ho₂O₃) or thuliumoxide (Tm₂O₃) doping concentration is 0.5–15 wt. %. If co-doped, thethulium/holmium concentration can range from 0.5 to 15 wt. %. Theformation of the gratings in passive silica fiber rather than the gainfiber reduces ASE noise, which improves linewidth. As will be describedwith reference to FIGS. 5 a and 5 b, thermal stabilization and vibrationisolation of the gain fiber and gratings and the use of a polarizationmaintaining (PM) fiber in the pump source further improve and maintainlinewidth in commercial applications.

A source of pump radiation 22, e.g. a single-mode or multi-mode laserdiode, illuminates gain fiber 12 at a wavelength, typically 800 nm,within the absorption band. As shown in FIG. 2, the absorption band 24 aof triply ionized thulium occurs from 750 to 820 nm. The pump wavelengthcan be around 1.2 μm and 1.7 μm to excite Thulium from ³H₆ state to ³H₅and ³F₄ levels. The absorption band 24 b of triply ionized holmiumoccurs from 1800 to 2100 nm. The pump wavelength can be around 1800 μmand 2100 μm to excite Holmium from ⁵I₈ to ⁵I₇ levels. Typically oneexcites thulium ions, which in turn transfers energy to the holmiumions. The pump wavelength can also be shifted to other wavelengths toexcite Yb and Er when these ions are doped into the host. Pumping of thedoped glass populates the thulium upper level creating a populationinversion. Spontaneous emission catalyzes the stimulated emission of thethulium (holmium) ions in the upper level over a range of 1800 to 2000nm (1900 to 2200 nm). The emission peak of thulium (holmium) occurs at awavelength of approximately 1800 nm (2080 nm).

Laser emission occurs when the total gain produced by stimulatedemission overcomes the losses in the cavity. The gain profile of thuliumand holmium and the geometry of the resonant cavity create preferentialfeedback so that laser emission only occurs at one or more discretewavelengths. As shown in FIG. 3, an ultra-short cavity, less than 5 cmand preferably less than 3 cm, produces a mode spacing Δν that is atleast comparable to and preferably larger than the linewidth 30(full-width half-max of the peak) of the narrowband grating (less than0.07 nm), which together with the gain spectrum 31 limits laser emissionto a single longitudinal mode 32. Therefore, a very high gain per unitlength is needed in order to achieve single frequency fiber laseroperation if a linear cavity is used. The gain per unit length requiredhere typically exceeds normally achievable value. Glasses with highdoping concentration and fibers with high gain per unit length arerequired. In addition, a fusion splicing technique with a short workingdistance between the active fiber and the grating fibers is needed.

To achieve single-frequency performance the glass host must support highThulium and Holmium doping concentrations to realize the necessary gainper unit length. The doping concentrations are at least 0.5 wt. % andtypically about 1–8 wt. %. Therefore, glasses with good rare-earth ionsolubility must be selected.

Suitable oxide-based multi-component laser glasses contain either agermanate-oxide (GeO₂) or tellurite-oxide (TeO₂) network former, one ormore glass network modifiers including alkaline-earth oxides andtransition metal oxides MO such as BaO, CaO, MgO, SrO, ZnO, PbO andmixtures thereof and/or alkali metal oxides R₂O such as Na₂O, Li₂O andK₂O and mixtures thereof, and one or more glass network intermediatorsL₂O₃ such as Y₂O₃, La₂O₃, Al₂O₃, B₂O₃, WO₃ and mixtures thereof. Themodifier modifies the glass network, thereby reducing its meltingtemperature and creating additional dopant sites. The intermediatorbridges some of the bonds in the network thereby increasing thenetwork's strength and chemical durability without raising the meltingtemperature appreciably. The glass host composition by weight percent is30 to 85% network former, 2 to 50% modifier including 0–10% MO and 0–20%R₂O, and 0.2 to 50% intermediator. Typically, the intermediator is atleast 2 wt. % and suitably 10 wt. % and the total modifier is at least 2wt. % and suitably 25 wt. %.

As shown in FIG. 4, the fiber core 40 is made up of the glass host dopedwith high concentrations of thulium or holmium oxide or mixturesthereof. The cladding layer(s) 42 are formed from the same glass host,although the exact composition may vary somewhat from the core glass,and are typically undoped. Numerous glass compositions werecharacterized for thermal properties (crystallization, expansioncoefficient, transition and softening temperatures, core-to-cladsimilarity), chemical durability, ability to host high Tm (Ho) dopingconcentrations without quenching, spectroscopic properties (maximumemission cross-section at 2.0 microns), linewidth (maximum breadth at2.0 microns) and refractive index to determine a range of wt. % for eachingredient that are acceptable. Suitable glass compositions forgerminate host glasses are illustrated in Tables 1 and 2 and telluritehost glasses are illustrated in Table 3, respectively.

TABLE 1 CG-S-3 GeO Al₂O₃ BaO Na₂O Ho₂O₃ Total Wt % 61.82 7.36 17.29 6.716.82 100 DG-S-1 GeO Al₂O₃ BaO Na₂O La₂O₃ Total Wt % 62.41 7.43 17.466.77 5.93 100 DG-S-4 GeO Al₂O₃ BaO Na₂O La₂O₃ Total Wt % 62.41 7.4317.46 6.77 5.93 100 DG-S-6 GeO Al₂O₃ BaO Na₂O La₂O₃ Total Wt % 62.417.43 17.46 6.77 5.93 100

TABLE 2 GeO₂ Al₂O₃ BaO CaO MgO Na₂O Li₂O K₂O La₂O₃ Total CG-S-1 65.508.00 12.50 12.00 2.00 100.00 G-S-1 65.50 8.00 24.50 0.00 2.00 100.00G-S-2 65.50 8.00 18.00 6.50 2.00 100.00 G-S-3 65.50 8.00 12.50 12.002.00 100.00 G-S-4 65.50 8.00 6.50 18.00 2.00 100.00 G-S-5 65.50 8.000.00 24.50 2.00 100.00 G-S-6 65.50 8.00 6.50 18.00 2.00 100.00 G-S-765.50 6.00 6.50 20.00 2.00 100.00 G-S-8 65.50 4.00 6.50 22.00 2.00100.00 G-S-9 65.50 2.00 6.50 24.00 2.00 100.00 G-S-10 61.50 6.00 6.5024.00 2.00 100.00 G-S-11 57.50 6.00 6.50 28.00 2.00 100.00 G-S-12 65.506.00 6.50 20.00 2.00 100.00 G-S-13 65.50 6.00 6.50 20.00 2.00 100.00G-S-14 61.50 6.00 6.50 4.00 20.00 2.00 100.00 G-S-15 61.50 6.00 6.5024.00 2.00 100.00 G-S-16 61.50 6.00 6.50 24.0 2.00 100.00 G-S-17 57.5 610.5 24 2 100.00 G-S-18 53.5 6 14.5 24 2 100.00 G-S-19 60 6 8 24 2100.00 G-S-20 60 6 6.5 25.5 2 100.00 G-S-21 60 6 6.5 20 5.5 2 100.00G-S-22 60 6 4.5 2 20 5.5 2 100.00 G-S-23 60 7.5 6.5 24 2 100.00

TABLE 3 Na₂O Al₂O₃ B₂O₃ TeO₂ Tm₂O₃ Total wt % TB-T1 7.4  8.11 8.31 76.18100 TB-T2 7.38 8.09 8.29 75.99 0.25 100 TB-T3 7.36 8.07 8.27 75.8 0.5100 TB-T4 7.25 7.95 8.15 74.69 1.96 100 TB-T5 7.18 7.87 8.07 73.96 2.92100 TB-T6 7.05 7.72 7.91 72.55 4.77 100 TB-T7 6.92 7.58 7.76 71.2 6.54100 K₂O Li₂O WO₃ TeO₂ Tm₂O₃ Total wt % TW-T1 4.33 1.38 35.46 58.58 0.25100 TW-T2 2.33 2.58 35.67 58.92 0.5 100 TW-T3 2.31 2.57 35.49 58.63 1100 TW-T4 2.29 2.54 35.14 58.05 1.98 100

Based on this empirical data, a first exemplary embodiment of a suitablecore laser glass may comprise by weight percent:

45 to 70 GeO₂;

0.5 to 20 L₂O₃ selected from Al₂O₃, B₂O₃ and La₂O₃ and mixtures thereof;

2 to 20 MO selected from BaO, CaO and MgO and mixtures thereof;

2 to 30 R₂O selected from Na₂O, Li₂O, and K₂O and mixtures thereof; and

0.5–15 Ho₂O₃, Tm₂O₃ and mixtures thereof.

Based on this empirical data, a second exemplary embodiment of asuitable core laser glass may comprise by weight percent:

45 to 85 TeO₂;

0.5 to 15 L₂O₃ selected from Al₂O₃, B₂O₃ and La₂O₃ and mixtures thereof;

0.5 to 20 R₂O is selected from Na₂O, Li₂O, and K₂O and mixtures thereof;and

0.5–15 Ho₂O₃, Tm₂O₃ and mixtures thereof.

Based on this empirical data, a third exemplary embodiment of a suitablecore laser glass may comprise by weight percent:

45 to 85 TeO₂;

10 to 45 WO₃; and

0.5 to 20 R₂O selected from Na₂O, Li₂O, and K₂O and mixtures thereof;and

0.5–15 Ho₂O₃, Tm₂O₃ and mixtures thereof.

In another embodiment, the gain fiber 12 is a polarization maintaining(PM) fiber, which reduces the noise level in the laser and improvesoutput power stability. As a result of their specific local environment,the active ions experience an anisotropy that leads to polarizationdependent gain, which means that the gain spectra for two differentpolarization components are not the same. Combining polarizationselective feedback with a polarization maintaining active fiber reducesthe low frequency noise that results from random polarizationfluctuations inside the laser cavity.

The subclass of multi-component glasses has a much lower softeningtemperature (<600° C.) than silica (>1200° C.), which greatly simplifiesthe fiber drawing process and supports higher doping concentrations butcomplicates the process of fusion splicing to silica fiber. A qualityfusion splice should exhibit low optical loss (<0.3 dB), low backreflection loss (<−50 dB) and good tensile strength (>100 g). A standardsilica-to-silica fusion splice degrades the multi-component fiber.Instead an asymmetric process that softens only the multi-componentfiber is employed as described in co-pending U.S. application Ser. No.09/963,727 entitled “Method of Fusion Splicing Silica Fiber withLow-Temperature Multi-Component Glass Fiber” filed on Sep. 26, 2001,which is incorporated by reference. To reduce back-reflection, an anglesplicing process in which the silica fiber is angle cleaved, themulti-component fiber square cleaved and a matched angle formed in-situmay be employed as described in co-pending U.S. application Ser. No.10/374,001 entitled “Method of Angle Fusion Splicing Silica Fiber withLow-Temperature Non-Silica Fiber” filed on Feb. 25, 2003 the entirecontents of which are incorporated by reference.

As shown in FIGS. 6 a and 6 b, the single mode 2 μm laser is placedinside a package 50 that provides thermal and vibration isolation. Thefiber chain 52 is placed in a mounting fixture 54 having first andsecond thermally isolated sections 56 and 58 for supporting thenarrowband and broadband fiber gratings 14 and 16 respectively. The gainfiber is supported in either the first or second section or in a thirdisolated section (not shown in this embodiment). Resistive heaters 60thermistors 61 are mounted on the first and second sections andindependently controlled to match the wavelengths of narrowband andbroadband gratings.

The mounting fixture 54 is connected to the laser external package 50through connectors 62 made of compliant material with relatively smallYoung modulus and a small thermal conductivity coefficient. The mountingfixture 54 containing the fiber chain and more specifically the firstand second thermally isolated sections 56 and 58 are independentlyheated above room temperature and temperature stabilized. The connectionto the external package 50 with the compliant connectors 62 providessimultaneously good mechanical and thermal stability to the laseritself. This is necessary for stable laser output.

As described above in connection with FIG. 1, the optical cavity must bepumped to induce lasing. As shown in FIG. 6, a single-mode semiconductorpump diode 22 includes a semiconductor chip 70 and a grating 72separated by about a meter of passive fiber 74. The grating locks thepump's output to a wavelength. In most applications, the OTS pump diodeis adequate. Since, however, very narrow linewidth lasers have a numberof applications in sensing, in particular acoustic sensing, it isimportant to ensure very low frequency and phase noise at lowfrequencies. The phase noise is particularly sensitive to polarizationfluctuations in the pump fiber. Single mode semiconductor pump lasersemit highly polarized light. The polarization of the pump light is,however, sensitive to birefringence fluctuations in the pump fiber if PMfiber is not employed. The fiber 74 that leads from the diode 70 to thefiber laser is typically longer than 1 m and any vibration and acousticpickup in this fiber leads to small changes in the pump lightpolarization. Due to the anisotropy of the active ions, this leads toadditional noise in the fiber laser output. The effect is morepronounced in the phase noise as in the intensity noise. Ronnekleiv hasalready pointed out that this vibration and pressure sensitivity couldbe largely reduced if one would use a depolarized pump source.[“Frequency and Intensity Noise of Single Frequency Fiber Bragg GratingLasers”, by Erlend Ronnekleiv, Optical Fiber Technology, 7, 206–235(2001)—page 227, second paragraph]. Placing a depolarizer between thehighly-polarized pump laser and the fiber laser is one way to reduce theacoustic pickup in the lead fiber.

Using polarization maintaining (PM) fiber 74 from the pump diode 70 tothe fiber laser avoids the costly depolarizer and has the same effect.Due to the birefringence in polarization maintaining fiber, thepolarization state of the pump light will not change when the fiber issubject to mechanical vibrations or acoustic pressure waves. Thepolarization whose stimulated emission cross section of the gain fiberis higher is aligned to the orientation of the operating polarization ofthe narrow-band fiber Bragg grating. Experiments have shown that usingPM fiber in the pump lead greatly reduces the phase noise of the fiberlaser output and the sensitivity to low frequency external noise.

While several illustrative embodiments of the invention have been shownand described, numerous variations and alternate embodiments will occurto those skilled in the art. Such variations and alternate embodimentsare contemplated, and can be made without departing from the spirit andscope of the invention as defined in the appended claims.

1. A fiber laser, comprising: A gain fiber less than 5 cm in length including, A core formed from a glass host selected from germanate, tellurite or fluoride, said glass host being doped with 0.5–15 wt. % of thulium or holmium oxide or mixtures thereof; and A cladding formed from the same glass host; A narrowband grating having a linewidth and a broadband grating at opposite ends of the fiber that define an optical resonant cavity; and A source of pump radiation that illuminates the fiber to excite the dopant ions and provide gain; the length of the resonant cavity producing a mode spacing that is comparable with the narrowband grating's linewidth so that the dopant ions lase at a single longitudinal mode, said fiber laser outputting a single-frequency signal having a center wavelength between approximately 1.8 μm and 2.1 μm with a linewidth less than 100 kHz.
 2. The fiber laser of claim 1, wherein the glass is doped with 1–8 wt. % thulium or holmium oxide.
 3. The fiber laser of claim 1, wherein the linewidth of the single-frequency signal is less than 10 kHz.
 4. The fiber laser of claim 1, wherein the narrowband and broadband gratings are formed in sections of passive silica fiber that are fusion spliced to the ends of the gain fiber.
 5. The fiber laser of claim 1, wherein the pump includes a section of polarization maintaining fiber.
 6. The fiber laser of claim 1, wherein the multi-component glass includes the following ingredients by weight percentages, a network former of 30 to 85 percent, where the network former is selected from germanate-oxide (GeO₂) or tellurite-oxide (TeO₂), a glass intermediator L₂O₃ of 0.5 to 50 percent, where L₂O₃ is selected from Al₂O₃, B₂O₃, Y₂O₃, La₂O₃, WO₃ and mixtures thereof, and a glass modifier of 2 to 50 percent including (a) MO of 0 to 20 percent, where MO is selected from BaO, BeO, MgO, SrO, CaO, ZnO, PbO, and mixtures thereof, and (b) R₂O of 0 to 20 percent, where R₂O is selected from Na₂O, Li₂O and K₂O and mixtures thereof.
 7. The fiber laser of claim 6, wherein the glass includes by weight percentages, 45 to 70 GeO₂; 0.5 to 20 L₂O₃; 2 to 20 MO; and 2 to 30 R₂O, wherein L₂O₃ is selected from Al₂O₃, B₂O₃ and La₂O₃ and MO is selected from BaO, CaO and MgO.
 8. The fiber laser of claim 6, wherein the glass includes by weight percentages, 45 to 85 TeO₂; 0.5 to 15 L₂O₃; and 0.5 to 20 R₂O, wherein L₂O₃ is selected from Al₂O₃, B₂O₃ and La₂O₃.
 9. The fiber laser of claim 6, wherein the glass includes by weight percentages, 45 to 85 TeO₂; 10 to 45 WO₃; and 0.5 to 20 R₂O.
 10. The fiber laser of claim 1, wherein the gain fiber is less than 5 cm in length.
 11. The fiber laser of claim 1, wherein the gain fiber is a polarization maintaining (PM) fiber.
 12. The fiber laser of claim 11, wherein the polarization whose stimulated emission cross section of the gain fiber is higher is aligned to the orientation of the operating polarization of the narrow-band fiber Bragg grating.
 13. The fiber laser of claim 11, wherein the mode spacing is greater than 0.07 nm.
 14. The fiber laser of claim 1, wherein the narrowband grating has a linewidth less than 0.07 nm and the broadband grating has a linewidth between 0.07 nm and 0.4 nm.
 15. A fiber laser, comprising: A gain fiber less than 5 cm in length including, A cladding formed from fluoride, germanate or tellurite glass host; and A single mode core formed from the same glass host doped with 0.5–15% thulmium or holmium oxide or mixtures thereof; A passive silica fiber having a narrowband grating formed therein and fused at one end of the gain fiber; A passive silica fiber having a broadband grating formed therein and fused at the other end of the gain fiber; and A source of pump radiation that illuminates the fiber so that said fiber outputs a single-frequency signal having a center wavelength at approximately 2 μm.
 16. The fiber laser of claim 15, wherein the glass is doped with 1–8 wt. % thulmium or holmium oxide.
 17. The fiber laser of claim 15, wherein the glass includes by weight percentages, a network former of 30 to 85 percent, where the network former is selected from germanate-oxide (GeO₂) or tellurite-oxide (TeO₂), a glass intermediator L₂O₃ of 0.5 to 50 percent, where L₂O₃ is selected from Al₂O₃, B₂O₃, Y₂O₃, La₂O₃, WO₃ and mixtures thereof, and a glass modifier of 2 to 50 percent including (a) MO of 0 to 20 percent, where MO is selected from BaO, BeO, MgO, SrO, CaO, ZnO, PbO, K₂O, Li₂O and mixtures thereof, and (b) R₂O of 0 to 20 percent, where R₂O is selected from Na₂O, Li₂O and K₂O and mixtures thereof.
 18. The fiber laser of claim 15, wherein the source of pump radiation and/or the gain fiber includes a polarization maintaining (PM) fiber.
 19. The fiber laser of claim 15, wherein the narrowband grating has a linewidth less than 0.07 nm and the broadband grating has a linewidth between 0.07 nm and 0.4 nm, said narrowband and broadband gratings forming a resonant cavity less than 5 cm in length with a mode spacing that is greater than 0.07 nm so that said single-frequency signal has a linewidth less than 10 kHz.
 20. A fiber laser, comprising: A gain fiber less than 5 cm in length including, A core formed from an oxide-based multi-component glass host including by weight percentage a network former of 30 to 85 percent selected from germanate-oxide (GeO₂) or tellurite-oxide (TeO₂), a glass intermediator L₂O₃ of 0.5 to 50 percent selected from Al₂O₃, B₂O₃, Y₂O₃, La₂O₃, WO₃ and mixtures thereof, and a glass modifier of 2 to 50 percent including (a) MO of 0 to 20 percent selected from BaO, BeO, MgO, SrO, CaO, ZnO, PbO, K₂O, Li₂O and mixtures thereof, and (b) R₂O of 0 to 20 percent selected from Na₂O, Li₂O and K₂O and mixtures thereof, said glass host being doped with 0.5–15 weight percent thulmium or holmium oxide or mixtures thereof; and A cladding formed from the same glass host; A narrowband grating at one end of the fiber; A broadband grating at the other end of the fiber; and A source of pump radiation that illuminates the fiber so that the dopant oxide ions lase at a single longitudinal mode and said fiber outputs a single-frequency signal having a center wavelength at approximately 2 μm.
 21. The fiber laser of claim 20, wherein the narrowband and broadband gratings are formed in sections of passive silica fiber that are fusion spliced to the ends of the gain fiber.
 22. The fiber laser of claim 21, wherein the narrowband grating has a linewidth less than 0.07 nm and the broadband grating has a linewidth between 0.07 nm and 0.4 nm, said narrowband and broadband gratings forming a resonant cavity less than 5 cm in length with a mode spacing that is greater than 0.07 nm so that said single-frequency signal has a linewidth less than 10 kHz.
 23. The fiber laser of claim 20, wherein the single-mode signal has a linewidth of less than 10 kHz.
 24. The fiber laser of claim 20, wherein the glass is doped with 1–8 wt. % thulium or holmium oxide.
 25. The fiber laser of claim 20, wherein the glass includes by weight percentages, 45 to 70 GeO₂; 0.5 to 20 L₂O₃; 2 to 20 MO; and 2 to 30 R₂O, wherein L₂O₃ is selected from Al₂O₃, B₂O₃, La₂O₃ and MO is selected from BaO, CaO and MgO.
 26. The fiber laser of claim 20, wherein the glass includes by weight percentages, 45 to 85 TeO₂; 0.5 to 15 L₂O₃; and 0.5 to 20 R₂O, wherein L₂O₃ is selected from Al₂O₃, B₂O₃, La₂O₃.
 27. The fiber laser of claim 20, wherein the glass includes by weight percentages, 45 to 85 TeO₂; 10 to 45 WO₃; and 0.5 to 20 R₂O. 